Creating integrated circuit capacitance from gate array structures

ABSTRACT

Techniques for using gate arrays to create capacitive structures within an integrated circuit are disclosed. Embodiments comprise placing a gate array of P-type field effect transistors (P-fets) and N-type field effect transistors (N-fets) in an integrated circuit design, coupling drains and sources for one or more P-fets and gates for one or more N-fets to a power supply ground, and coupling gates for the one or more P-fets and the drains and sources for one or more N-fets to a positive voltage of the power supply. In some embodiments, source-to-drain leakage current for capacitive apparatuses of P-fets and N-fets are minimized by biasing one or more P-fets and one or more N-fets to the positive voltage and the ground, respectively. In other embodiments, the capacitive structures may be implemented using fusible elements to isolate the capacitive structures in case of shorts.

CROSS-REFERENCES TO RELATED APPLICATIONS

Pursuant to 35 USC §120, this continuation application claims priorityto and benefit of U.S. patent application Ser. No. 11/337,010, entitled“METHODS AND APPARATUSES FOR CREATING INTEGRATED CIRCUIT CAPACITANCEFROM GATE ARRAY STRUCTURES”, filed Jan. 20, 2006, the disclosure ofwhich is incorporated herein in its entirety for all purposes.

FIELD

The present invention generally relates to the field of integratedcircuits. More particularly, the present invention relates to providingcapacitance to integrated circuits from gate array structures formed ina semiconductor substrate.

BACKGROUND

Today, integrated circuits (“ICs”) may contain millions of transistorson a single chip, with many critical circuit features havingmeasurements in the deep sub-micron range. ICs are fabricated layer bylayer on a semiconductor substrate. Using techniques known in the art ofsemiconductor fabrication, metal-oxide-semiconductor (“MOS”)transistors, bipolar transistors, diodes and other devices arefabricated and combined to form an IC on a substrate. Typically,portions of some of the devices and some interconnections are frequentlyformed using one or more levels of polysilicon. For example, a MOStransistor gate electrode and a resistor may be fabricated from a layerof polysilicon.

The IC layers are fabricated through a sequence of pattern definitionsteps that are mixed with other process steps such as oxidation,etching, doping, and material deposition. One or more metal layers arethen deposited on top of the base layers to form conductive segmentsthat interconnect IC components. Formation of the metallization layersover the substrate facilitates interconnection of the transistors toform more complex devices such as NAND gates, inverters, and the like.These metallization layers may also be used to provide power supplyground (V_(SS)) and power supply voltage (V_(DD)) to such IC devices.

The metallization layers utilize lines, contacts, and vias tointerconnect the transistors in each of the cells as well as tointerconnect the cells to form the integrated circuit, such as aprocessor, state machine, or memory. Lines in adjacent vertical layersoften run perpendicular to one another, the adjacent vertical layersseparated by a non-conductive passivation layer such as, e.g., siliconoxide. The silicon oxide is etched to form the vias, which interconnectthe lines of various metallization layers in accordance with the circuitdesign. Inputs and outputs of the integrated circuit are brought to asurface with vias to bond the circuits with pins of a chip package. Thechip package typically includes an epoxy or ceramic that encloses theintegrated circuit to protect the circuit from damage and pins tofacilitate a connection between the inputs and outputs of the integratedcircuit and, e.g., a printed circuit board.

As stated above, the finished product IC may contain millions oftransistors. Many of these transistors may operate with rapid switchingrates. The operation of low-power, high-speed integrated circuits may beaffected by these rapid switching rates. The extremely rapid switchingrates of the transistor and other discrete components that make upintegrated circuits typically cause current transients in the powerbuses of the integrated circuits. These current transients may last forseveral nanoseconds. Unfortunately, the power supply for the circuit mayrequire much more time, such as several microseconds, to compensate forthe transient currents drawn from the power bus by the discretecomponents. As a consequence, these transient currents from the powerbus cause noise in power supply rails. Low gate threshold voltages ofthe various discrete components in ICs require the power supply bus todeliver a stable voltage, with minimum voltage level variations.Consequently, power supply bus stability, in terms of current responseand voltage level fluctuation, is a significant issue in the design ofan integrated circuit.

The conventional approach for stabilizing the power supply bus is toinsert decoupling capacitors between the power supply bus and the ICcircuit elements. Decoupling capacitors placed near power consumingcircuits tend to stabilize, or smooth out, voltage variations byutilizing the charge stored within the decoupling capacitor. The storedcharge may be thought of as a local power supply, providing power duringthe times that the discrete components switch rapidly. The net resultbeing that the decoupling capacitors help mitigate the effects ofvoltage noise induced onto the system power supply bus.

Generally, IC designers may incorporate decoupling capacitance directlyon the IC by only a few means, such as using thin oxide capacitance,metal-to-metal capacitance, and junction capacitance. While thin oxidecapacitance, such as that associated with device gate oxide, offers thehighest capacitance per unit area, it also has higher gate leakage andtends to lower IC fabrication yield due to gate oxide shorts than theother capacitance options.

Today, gate oxide thickness of ICs is merely a few layers of atoms andis approaching fundamental limits. For example, in a typicalcomplimentary metal-oxide-semiconductor (CMOS) device the gate oxidethickness is often less than two nanometers. Consequently, such a smallthickness makes the device susceptible to the effects of gate leakagecurrent from other circuit components coupled with the device. Oneexample is gate tunneling leakage current generated by one or moredecoupling capacitors that couple the device to the power bus. Anotherexample is a gate oxide defect causing a short between two plates. Smallholes, or other defects in the oxide, often result in gate oxide leakagecurrents.

Typically, thin oxide structures used to create decoupling capacitanceare field effect transistor (fet) based devices with wide device widthsand relatively long channels. These structures are generallyinterspersed with the logic gates of the IC circuit design. Vacantregions of an IC may also be populated with gate array back fill cells,to allow designers to satisfy pattern density requirements and allowengineers to incorporate design changes. Unfortunately, the precise needfor decoupling capacitance is not known until the IC circuit design hasbeen placed and routed. Decoupling capacitance needs may disrupt thedesign process, requiring designers and engineers to rearrange theinitial circuit placement to accommodate insertion of the decouplingcapacitance structures.

Using deep sub-micron technology to design ICs, combined with the strictneed for minimizing leakage currents and the need to have adequateamounts of decoupling capacitance, presents circuit designers withnumerous design problems. What is needed is a new apparatus for creatingdecoupling capacitance using gate array cells while minimizing yieldlosses due to gate oxide defects, leakage current, and placementdisruption.

SUMMARY

The problems identified above are in large part addressed by methods andapparatuses for creating integrated circuit capacitance from gate arraystructures. One embodiment comprises a method for creating capacitance,such as decoupling capacitance, from a gate array containing numerousP-fets and N-fets in an integrated circuit using one or more modifiedgate array cells. The method generally involves placing unmodified cellsin a circuit design, coupling the gates of the P-fets to a positivevoltage power supply, and coupling the sources and drains of the P-fetsto a ground of the voltage power supply to create a capacitivestructure. Capacitive structures may also be created and isolated usinga number of the gate array N-fets in other embodiments of the method.

In another embodiment, creating the integrated circuit capacitance mayinvolve using N-fets in a P-well. In other embodiments, the integratedcircuit capacitance may involve using P-fets in an N-well on asubstrate. The capacitive N-fets structures may be created by coupling anumber of the gate array N-fet sources and drains to a positive voltagepower supply and coupling the number of corresponding gates of theN-fets to ground. Creating the capacitive P-fet structures may besimilarly created by coupling a number of P-fet source and drainconnections to a power supply ground, connecting the number of P-fetgates and the N-well to a positive voltage power supply, and couplingthe substrate to the power supply ground.

In another embodiment, the capacitive P-fet structures may be isolatedfrom the rest of the integrated circuit by placing one or more P-fets,with the transistors being turned off by appropriate biasing of theirgates, in series with the capacitive P-fet structures. Inserting theisolating P-fets in this manner may help reduce the negative effects ofgate oxide defects and leakage currents.

In further embodiments, the capacitive N-fet structures may be isolatedfrom the rest of the integrated circuit by placing one or more N-fets,with the transistors being turned off by coupling their gates to ground,in series with the capacitive N-fet structures. In even furtherembodiments, the capacitive N-fet and P-fet structures may beinterconnected in a manner to create a fusible link that may protect thestructures from manufacturing shorts in gate array structures.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the invention will become apparent upon reading thefollowing detailed description and upon reference to the accompanyingdrawings in which, like references may indicate similar elements:

FIG. 1A depicts an ASIC system with a CPU, RAM, I/O blocks, D/A and A/Dconverters, and various gate arrays;

FIG. 1B depicts an ASIC system with two gate arrays modified to createcapacitance;

FIG. 1C illustrates how a semiconductor may be configured to function asa capacitor;

FIG. 1D depicts schematics for creating a capacitor from an N-fet in anN-well;

FIG. 2A depicts an alternate schematic for creating a capacitor from anN-fet in a P-well;

FIG. 2B depicts another alternative schematic for creating a capacitorfrom an N-fet in a P-well;

FIGS. 3A-3B illustrates two configurations and associated schematics forcreating capacitive structures from using P-fets in an N-well;

FIG. 4 shows a schematic for a gate array that may be configured tocreate capacitance for an integrated circuit;

FIG. 5 illustrates a physical arrangement of gate array elements thatmay be configured to create capacitance;

FIG. 6 shows a schematic of a capacitive structure comprising acollection of P-fets and a collection of N-fets that are isolated by apair of P-fets and a pair of N-fets, respectively;

FIG. 7 illustrates a physical arrangement of a capacitive structurecomprising a collection of P-fets and a collection of N-fets that areisolated by a pair of P-fets and a pair of N-fets, respectively;

FIG. 8 shows a schematic of a capacitive structure comprising acollection of P-fets and a collection of N-fets that are isolated by twopairs of P-fets and two pairs of N-fets, respectively, each pair beingconnected in series;

FIG. 9 illustrates a physical arrangement of a capacitive structurecomprising a collection of P-fets and a collection of N-fets that areisolated by two pairs of P-fets and two pairs of N-fets, respectively,each pair being connected in series;

FIG. 10 depicts a flowchart of a method for creating capacitance fromN-fets and P-fets in a gate array structure; and

FIG. 11 depicts a flowchart explaining a process of how field effecttransistors may be connected for storing and discharging energy in anintegrated circuit.

DETAILED DESCRIPTION OF EMBODIMENTS

The following is a detailed description of example embodiments of theinvention depicted in the accompanying drawings. The example embodimentsare in such detail as to clearly communicate the invention. However, theamount of detail offered is not intended to limit the anticipatedvariations of embodiments; but, on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the present invention as defined by the appendedclaims. The detailed descriptions below are designed to make suchembodiments obvious to a person of ordinary skill in the art.

Generally speaking, methods and apparatuses for creating integratedcircuit (IC) capacitance from gate array structures are discussed. Newtechniques for creating decoupling capacitance from gate arrays andstructures within gate array cells formed in a semiconductor substrateof an integrated circuit are discussed. Embodiments comprise variousgate array elements configured in different manners to providecapacitance for an integrated circuit. In one embodiment, numerousstructures within a gate array cell, which could be configured to createN-type field effect transistors (N-fets) in a P-type diffusion well(P-well), are instead configured to create a capacitive element that canbe used as a decoupling capacitor for the integrated circuit.Additionally, the embodiment may also contain numerous other structureswithin the gate array cell, which could otherwise be configured tocreate P-type field effect transistors (P-fets) in an N-type diffusionwell (N-well), that are also configured to create a capacitive element.

In alternative embodiments, structures within a gate array cell areconfigured to create enhanced capacitive elements. These enhancedcapacitive elements may have the additional benefit of minimizing yieldlosses resulting from gate oxide defects. In even further alternativeembodiments, structures within a gate array cell are configured tocreate capacitive elements which may not only minimize yield losses fromgate oxide defects, but may also minimize drain-source leakageassociated with isolation devices.

While portions of the following detailed discussion describe manyembodiments enabling one to create new capacitive structures from gatearrays for decoupling use within integrated circuits, upon review of theteachings herein, a person of ordinary skill in the art will recognizethat the following invention may be practiced in a variety of ways, suchas creating capacitance for other integrated circuit needs, unrelated todecoupling. All methods of practicing the invention are interchangeablefor such different purposes. Further, while descriptions of embodimentsmay show how to create capacitance from polycrystalline silicon gatestructures, and silicon substrates, one of ordinary skill in the art mayuse clever metal arrangements to create additional capacitance to thecapacitive gate array structures. Additionally, one may substitute othermaterials in these structures when employed in accordance with thediscussed embodiments and perform substantially equivalent functions.

Using gate arrays, or uncommitted logic arrays, is one approach fordesigning and manufacturing application-specific integrated circuits(ASICS). Gate array structures may be prefabricated structures with noparticular function in which transistors, standard logic gates, andother active devices are placed at regular predefined positions andmanufactured on a wafer. Creation of a circuit with a specified functionmay be accomplished by adding metal interconnects to the prefabricatedgate array structures, allowing the function of the gate array to becustomized as needed.

Integrated circuit engineers and designers may generally placeuncommitted gate array cells at numerous places in a circuit design.Doing so may enable the engineers and designers to make design changesat a later stage in the manufacturing process without having to alterthe circuit mask and circuit die. The gate arrays may fill what wouldotherwise be empty substrate areas in order to satisfy manufacturingsurface spacing and pattern density requirements. As noted above, thesegate arrays may also be customized to add transistors, logic gates, andother circuit components, such as resistors and capacitors, to theintegrated circuit.

Turning to the drawings, FIG. 1A illustrates how a system 100 maybenefit from methods and apparatuses for creating capacitance from gatearray structures. As depicted in FIG. 1A, system 100 may be an ASICcreated using a semiconductor substrate 116. System 100 may be dividedinto numerous functional areas and comprise numerous components, such asa central processing unit (CPU) 106, random access memory 114, cache122, peripheral input-output 120, and an input-output (I/O) block 135.System 100 may comprise components for translating digital and analogsignals, such as an analog-to-digital (A/D) converter 137 and adigital-to-analog (D/A) converter 136. For example, system 100 may be anASIC for a cellular telephone, with A/D converter 137 and D/A converter136 translating analog signals to and from a speaker and a microphone ofthe cellular telephone.

System 100 may also comprise numerous gate arrays located in variousareas of the integrated circuit, such as gate array 112, gate array 126,as well as gate arrays 130, 131, and 115. Such gate arrays may be usedin the ASIC to perform simple computations or logic functions outsidethe CPU 106, working in conjunction with other blocks, such asperipheral input-output block 120 or I/O block 135. Additionally, someor all of such gate arrays may comprise uncommitted gate array cellsadded to enable engineers to make design changes or to fill emptyintegrated circuit surface areas.

Numerous I/O pads 118 may be located around the periphery ofsemiconductor substrate 116, providing connection terminals for outsidepower and signal lines to system 100. As depicted in FIG. 1A, a powersupply system voltage VDD 102 may be terminated on I/O pad 103 and bedistributed throughout system 100 by numerous metal traces. For example,metal trace 104 may supply system voltage VDD 102 to the CPU 106 andmetal trace 110 may supply system voltage VDD 102 to the D/A converter136. Likewise, a system ground 139 may be terminated and distributed tothe D/A converter 136 and the CPU 106 by metal trace 138 and metal trace132, respectively.

System 100 may have transistors that operate at very high frequencies,to the extent that operating system 100 at these frequencies causescurrent transients, or noise, in the voltage power supply lines, such asmetal traces 104, 110, 132, and 138. Additionally, this noise may be ofsuch magnitude or frequency to cause logic errors in one or more system100 components. For example, the noise may cause logic errors in asection 108 of the CPU 106 and in a section 128 of the D/A converter136. A circuit designer may stabilize the power supply and reduce thenoise by inserting decoupling capacitors near the affected ICcomponents, in parallel with the components. The designer mayincorporate decoupling capacitance directly in the IC by using one ormore existing unused gate array structures. In FIG. 1A gate array 112sits in close proximity to the CPU 106 while gate array 115 residesclose to the D/A converter 136, both of which may be used to createdecoupling capacitance.

FIG. 1B illustrates how previously uncommitted or partially unusedstructures within gate arrays 112 and 115 may be modified to createcapacitance, according to various embodiments, and help reduce noisecreated on power supply metal traces 104, 110, 138, and 132. Note thatgate array 112 may be modified to create a decoupling capacitor 111,connected in parallel to the power metal traces 104 and 132 by way ofnew metal traces 107 and 109, respectively. Creating such decouplingcapacitance may attenuate the problem-causing noise in the power beingsupplied to section 108 of the CPU 106. Similarly, gate array 115 may bearranged to provide decoupling capacitance 119 for power metal traces110 and 138 via new metal traces 113 and 129, respectively. Decouplingcapacitance 119 connected in this manner may reduce the effects of noisein section 128 of D/A converter 136.

Generally, power and ground distributions, similar to those depicted inFIG. 1A and FIG. 1B, may form an interleaved matrix of wiring and allowaccess to various circuit functions, including non-personalized orunconfigured gate array cells. Configuring a gate array cell toimplement a decoupling capacitor may require adding additional metal andcontacts to access an existing power grid. Additionally, somenon-personalized gate array cells may be amorphous, in that they areused to fill otherwise unused regions of a chip. Such non-personalizedgate array cells may be transformed into design for manufacturabilitydecoupling capacitors using back-end-of-line (BEOL) techniques. In otherwords, the non-personalized or previously unconfigured gate array cellsmay be modified to create capacitance at a very late stage in the designprocess.

In some embodiments, system 100 may be a CPU. In further embodiments,system 100 may comprise a microcontroller or another type of integratedcircuit. Additionally, in other embodiments single or multiple parts ofthe integrated circuit may benefit from capacitance created from one ormore gate arrays. Also, different embodiments may not necessarily havethe capacitance gate arrays located immediately adjacent to the partsrequiring capacitance. Gate array capacitors may be located throughoutthe system 100. In order to understand how such gate array capacitorsmay be configured to supply capacitance to an integrated circuit, weturn now to FIG. 1C.

FIG. 1C illustrates one way a simple semiconductor apparatus may beconfigured to function as a capacitor. In FIG. 1C, a P-type material 142and an N-type material 145 are coupled together, forming a P-N junction143. If a positive voltage is applied to a terminal 146 of the N-typematerial 145, the voltage being positive with respect to a voltage atterminal 141 of the P-type material 142, the P-N junction 143 becomesreversed-biased. In the reverse-biased state, charged particles in boththe P-type material 142 and N-type material 145 move away from the P-Njunction 143 and increase the size of the associated depletion region144. This condition may produce a high resistance between terminals 141and 146, allowing a relatively small amount of current flow.Consequently, this depletion region 144 may serve as an insulation gap,functioning similar to a layer of dielectric material between the platesof a capacitor, and store energy in the outermost portions of the P-typematerial 142 and N-type material 145 as illustrated in FIG. 1C.

The technique for creating capacitance in a semiconductor apparatusshown in FIG. 1C may be employed in various embodiments. Alternatively,in other embodiments, different techniques may be employed to createcapacitance in a semiconductor apparatus. For example, instead of simplyusing a two-terminal P-N junction, one embodiment may employ athree-terminal P-N-P device. Alternatively, another embodiment mayemploy a three-terminal N-P-N device. Similarly, other embodiments mayemploy various combinations of materials, such as polysilicon, metal,and oxide. A few variations of such combinations are discussed in thefollowing figures.

FIG. 1D illustrates an apparatus that may create capacitance from onesection of a semiconductor substrate. A decoupling capacitor 150 may beconstructed from an N-fet in an N-well 152. N-well 152 may be situatedin a P-well or substrate region 190, of a semiconductor wafer. Note thatP-well or substrate region 190 surrounds N-well 152 in FIG. 1D. Further,note that the N-fet in N-well 152 may not be a configuration employed insome gate array cell structures. P-fets may be crafted within N-wells,similar to N-well 152, and N-fets may be crafted in P-wells orsubstrates.

An N-type diffusion area 155, is biased to a power supply voltage V_(DD)153. Similarly, an N-type diffusion area 185, is also biased to powersupply voltage V_(DD) 153. Polysilicon material 160, which may beconsidered a gate connection for the N-fet, may be connected to a ground180. The P-well or substrate region 190 may also be connected to ground180. In other words, N-type diffusion areas 155 and 185 may be biased topower supply voltage V_(DD) 153 while polysilicon material 160 and theP-well or substrate region 190 are connected to the power supply ground.FIG. 1D also depicts what may be an analogous schematic representation195 of the N-fet structure in an N-well configured to create acapacitive structure.

Connecting one transistor in this manner creates capacitance 165 betweensource N-type diffusion area 155 and gate polysilicon material 160. Thissource-to-gate capacitance 165 may be referred to as C_(SG). Similarly,this configuration creates capacitance 170 between gate polysiliconmaterial 160 and N-well 152. This N-well-to-gate capacitance 170 may bereferred to as C_(NWG). Likewise, this arrangement creates capacitance175 between drain N-type diffusion area 185 and gate polysiliconmaterial 160. This drain-to-gate capacitance 175 may be referred to asC_(DG). This arrangement also creates capacitance 173 between N-well 152and the P-well or substrate region 190, which may be referred to asC_(NWPW). The total capacitance for the structure, therefore, may be thesum of the individual capacitive elements, orC_(TOTAL)=C_(SG)+C_(NWG)+C_(DG)+C_(NWPW).

For the sake of clarity, one may think of the biased N-type diffusionareas 155 and 185, together with N-well 152, as forming one plate of aparallel plate capacitor. The gate polysilicon material 160 may bethought of as forming a second plate of the parallel plate capacitor.Decoupling capacitor 150 may offer a relatively high capacitance perunit area, as compared with some other fet configurations, but may allowyield losses if any gate oxide defects cause shorts between the twoplates. Also, pin-holes or other defects in the oxide may result inundesirable gate oxide leakage currents.

N-fet structures in N-wells are not the only configurations that may beused to create decoupling capacitance for an integrated circuit. FIG. 2Aand FIG. 2B show alternate methods for creating thin oxide capacitivestructures by using gate array N-fet structures in P-well or substratematerials. FIG. 2A illustrates an apparatus that may create a capacitor200 from one N-fet section of a gate array cell, wherein an uncommittedbase structure for creating an N-fet is located in a P-well. P-wellregion 222 may be connected to a ground connection 216. Source N-typediffusion 204 and drain N-type diffusion 220 may be coupled to powersupply voltage V_(DD) connection 202. Like the source N-type diffusion204 and drain N-type diffusion 220, gate polysilicon 208 may be coupledwith power supply voltage V_(DD) connection 202. To the right ofcapacitor 200 is shown what may be a schematic representation 230 of theN-fet in a P-well capacitor 200.

While not shown for the sake of clarity, one should note that P-wellregion 222 actually surrounds source N-type diffusion 204 and drainN-type diffusion 220, which would serve as the channel material for theN-fet structure. For example, P-well region 222 fills the area below thetransistor elements, similar to the manner P-well or substrate region190 surrounds N-well 152 in FIG. 1D. This shorthand depiction is typicalfor FIGS. 1D through 3B.

When connected in the manner shown in FIG. 2A, the structure creates acapacitance 206 between the N-type diffusion 204 and P-well region 222.This capacitance 206 may be termed C_(SPW). The connected structure alsocreates a capacitance 210 between gate polysilicon 208 material andP-well region 222. This capacitance 210 may be referred to as C_(GPW).Similarly, between N-type diffusion 220 and P-well region 222, theconnected structure creates capacitance 214, which may be referred to asC_(DPW). The total capacitance for the capacitor 200 structure, in thisinstance, may be the sum of the individual capacitive elements, namelyC_(TOTAL)=C_(SPW)+C_(GPW)+C_(DPW).

Coupling gate polysilicon 208 with power supply voltage V_(DD)connection 202 may provide greater amounts of capacitance than othersimilarly connected structures, but such a configuration may besusceptible to gate oxide leakage currents. Another configuration forcreating a capacitor from a gate array N-fet structure situated in aP-well is depicted in FIG. 2B, wherein gate polysilicon 248 is insteadconnected to ground 256. This configuration may have the benefits ofminimizing the issues associated with gate oxide defects and reducinggate-to-body tunneling leakage. Similar to the configuration shown inFIG. 2A, capacitor 240 has source N-type diffusion 244 and drain N-typediffusion 262 coupled with power supply voltage V_(DD) by connections242 and 260, respectively. Also similar to FIG. 2A, capacitor 240 hasP-well material 264 coupled to ground 256. To the right of capacitor 240is shown what may be a schematic representation 270 of this particularN-fet in a P-well capacitor 240.

When gate polysilicon 248 is instead connected to ground 256, asconfigured in FIG. 2B, the apparatus develops a capacitance 246 betweensource N-type diffusion 244 and P-well material 264. Likewise, theapparatus develops a capacitance 254 between drain N-type diffusion 262and P-well material 264. The apparatus also develops capacitance 250 and252 between gate polysilicon 248 and the N-type diffusion elements 244and 262, respectively.

When configured in the manner depicted in FIG. 2B, capacitor 240 mayhave a total capacitance value (C_(TOTAL)) equal to the sum ofcapacitance 246 (C_(SPW)), capacitance 254 (C_(DPW)), capacitance 250(C_(SG)), and capacitance 252 (C_(DG)). In other words, the totalcapacitance may be found by using the formulaC_(TOTAL)=C_(SPW)+C_(DPW)+C_(SG)+C_(DG).

As alluded to above, gate array N-fet structures in P-wells are one ofthe gate array configurations that may be used to create decouplingcapacitance for an integrated circuit. Other gate array configurationsthat may be used to create capacitance may comprise P-fet structuressituated in N-wells. FIG. 3A and FIG. 3B show two alternate methods forcreating thin oxide capacitive structures utilizing gate array P-fetstructures in N-wells.

FIG. 3A illustrates a capacitor 300, wherein an uncommitted gate arraybase structure for creating a P-fet is located in an N-well 306. N-well306 is biased to V_(DD) by way of power supply connection 320, coupledto N-well 306 using N-type diffusion material 324. Similarly, sourceP-type diffusion material 304 and drain P-type diffusion material 326are coupled to V_(DD) by way of power supply connections 302 and 322,respectively. P-fet gate polysilicon 308 and substrate 328 are connectedto ground by way of ground connections 318 and 330, respectively. Suchcapacitor 300 may have an equivalent schematic representation 340,depicted to the right of capacitor 300 in FIG. 3A.

When configured in the manner depicted in FIG. 3A, capacitor 300 mayhave a total capacitance value (C_(TOTAL)) equal to the sum ofcapacitance 310 (C_(SG)), capacitance 314 (C_(DG)), capacitance 312(C_(NWG)), and capacitance 316 (C_(NWSX)). In other words, the totalcapacitance may be determined by using the formulaC_(TOTAL)=C_(SG)+C_(DG)+C_(NWG)+C_(NWSX).

In an alternative embodiment, a P-fet structure situated in an N-wellmay have its gate polysilicon connected to V_(DD), while the P-typediffusion materials for the drain and gates may be grounded. Such anarrangement for a capacitive structure 350 is shown in FIG. 3B. Again,P-fet components may be situated in an N-well 362, which may be biasedto V_(DD) with power supply connection 372 by way of N-type diffusionmaterial 376. Gate polysilicon material 356 may also be biased to V_(DD)with connection 364.

Source P-type diffusion material 354 and drain P-type diffusion material378 may be grounded by grounding connections 352 and 374, respectively.P-well or substrate base material 380 may also be grounded with groundconnection 382. Capacitive structure 350 may be considered to have anequivalent schematic 390, also depicted in FIG. 3B.

When connected as shown in FIG. 3B, capacitive structure 350 may have atotal capacitance value (C_(TOTAL)) equal to the sum of capacitance 358(C_(GS)), capacitance 366 (C_(GD)), capacitance 360 (C_(SNW)),capacitance 368 (C_(DNW)), and capacitance 370 (C_(NWSX)). In otherwords, the total capacitance for capacitive structure 350 may bedetermined by using the formulaC_(TOTAL)=C_(GS)+C_(GD)+C_(SNW)+C_(DNW)+C_(NWSX).

Obviously, capacitive structures may be created from gate arrays in avariety of different ways, including ways not already noted. Suchcapacitive structures may well come within the boundaries of the methodsdiscussed herein for the various embodiments. Additionally, while theaforementioned descriptions of the various apparatuses may show separateor independent connections to power supply voltages and power supplygrounds, the power supply connections may actually be derived from thesame power supply source. For example, while FIG. 3B depicts threeseparate power supply ground connections, namely ground connections 352,374, and 382, all three ground connections may actually be derived froma single common metal connection. Similarly, the two power supplyvoltage connections, namely V_(DD) with power supply connection 372 andV_(DD) connection 364, may both be derived from a common metalconnection.

Turning now to the next figure, FIG. 4 shows a schematic for anuncommitted or unconfigured gate array 400 that may be configured todeliver capacitance in numerous embodiments or, alternatively, may beconfigured to provide various logic gates. The uncommitted gate array400 may comprise a plethora of basic circuit components, such as P-fetsand N-fets. In FIG. 4, gate array 400 comprises a plethora of P-fets 410and a plethora of N-fets 440. The number of P-fets and N-fets may varygreatly in different embodiments. FIG. 4 shows relatively few P-fets andN-fets to aid in clearly describing and conveying the operation ofvarious embodiments.

Gate arrays may have functionally isolated, or totally independent,elements. Alternatively, gate arrays may contain functionally committed,or dependent, elements. Stated differently, gate arrays may affect or beaffected by adjacent cells, depending on the particular gate array cellconfigurations. Gate array 400 contains P-Fet 430 and Pfet 470, whichmay be physically located near the cell boundary for gate array 400.Similarly, gate array 400 contains N-fet 450 and N-fet 480, which mayalso be located near the gate array cell boundary. While gate array 400may be uncommitted, these four transistors may have functionallycommitted elements, in that each of them may have pre-defined ordedicated connections to adjacent circuit elements. Note that both P-fet430 and P-fet 470 are connected to a power supply V_(DD) 420. Likewise,both N-fet 450 and N-fet 480 are connected to a power supply ground 460.

FIG. 5 shows what may be the physical layout corresponding to theschematic for gate array 400, shown in FIG. 4. More specifically, FIG. 5depicts the physical components for an unconfigured gate array 500,comprising a collection of P-fets in an N-well 501 and a collection ofN-fets in a P-well 502. For the collection of P-fets in an N-well 501,there is a P-diffusion region 510, numerous polysilicon gate structures505, and a metal rail 520, connected to a V_(DD) power supply rail 515,traversing the P-diffusion region 510 and polysilicon gate structures505. The metal rail 520 supplies power to the P-diffusion region 510 atthe gate array cell 550 boundaries by way of contact 525 and contact560.

Similar to the collection of P-fets in an N-well 501, the collection ofN-fets in a P-well 502 comprise an N-diffusion region 545 situated orpositioned under a collection of polysilicon gate structures 555. Ametal rail 540 traverses the N-diffusion region 545 and the collectionof polysilicon gate structures 555 and couples power supply ground 535to the N-diffusion region 545 at the gate array cell 550 boundaries byway of contact 530 and contact 565.

As discussed and depicted in the schematic for gate array 400 of FIG. 4,gate array 500 depicted in FIG. 5 may have boundary P-fets and N-fetsfunctionally committed, with dedicated connections to adjacent circuitelements. That is to say, contact 525, contact 560, contact 530, andcontact 565 may comprise shared junctions, coupling the gate array cell550 to adjacent cells. Gate array cells in alternative embodiments maynot contain such shared junctions. Again, gate array 500 is an examplegate array that may be used to create a capacitance structure in severalembodiments, using BEOL techniques. In other words, a gate array similarto gate array 500 may be configured to create capacitance in anintegrated circuit at a very late stage in the design process, such asafter “final” design but before actual mass production of an integratedcircuit device.

Note that in the several preceding paragraphs, FIG. 5 was described asdepicting the physical components for an unconfigured gate array 500,comprising a collection of P-fets in an N-well 501 and a collection ofN-fets in a P-well 502. While such a description may be accurate forvarious embodiments, the references to P-fets and N-fets mayalternatively, and potentially more accurately, be described asdiffusion volumes in a bulk or substrate. For example, the N-wellmaterial for the collection of P-fets in an N-well 501 may be moreaccurately described as an N-type diffusion volume created in a bulkmaterial, or substrate, which may be P-well or a P-type diffusionvolume. Also, the P-diffusion region 510 may be alternatively termed aP-type diffusion volume, positioned inside the N-well. Similarly, thecollection of N-fets in a P-well 502 was described as comprising anN-diffusion region 545 in a base or substrate, the structures mayalternatively be described as having an N-type diffusion volume locatedin a P-type diffusion volume. Also, while the unconfigured gate array500 may be described as having a series of polysilicon gate structures505 and a collection of polysilicon gate structures, these structuresmay alternatively be described as simply polysilicon materials. Suchalternative descriptions may also be used in the discussions for FIG. 7and FIG. 9, which follow.

FIG. 6 illustrates a schematic representation of a gate array 600configured as a capacitive device. In this embodiment, a collection ofP-fets 640 and a collection of N-fets 650 are arranged in a manner thatmay reflect how they may appear in a corresponding physical layout ofgate array 700, shown in FIG. 7. In gate array 600 the drain connectionsfor boundary P-fet 615 and boundary P-fet 660 are coupled to powersupply V_(DD) 610. Likewise, the source connections for boundary N-fet620 and boundary N-fet 670 are coupled to a power supply ground 630.

FIG. 6 further illustrates that the collection of P-fets 640 may allhave their source and drain connections coupled to power supply ground630, the exceptions being the boundary P-fets 615 and 660. Conversely,the gates for the collection of P-fets 640 are all connected to powersupply V_(DD) 610.

The collection of N-fets 650 may all have their source and drainconnections coupled to power supply V_(DD) 610, the exceptions againbeing boundary N-fet 620 and boundary N-fet 670. Conversely, the gatesfor the collection of N-fets 650 may all be coupled to power supplyground 630. Configuring the inside P-fets and N-fets in this manner mayallow the device to store charges and function as a capacitive devices,similar to the configuration shown in FIG. 2B and FIG. 3B.

Capacitance for this gate array device may be derived from storingcharges in the underlying N-diffusion and P-diffusion regions, orvolumes. The charges are figuratively sandwiched between the transistorgates, which function as one plate of a capacitor held at one potential,while the transistor source and drain materials serve as the othercapacitor plate, held at the opposite potential. Worth emphasis for thisembodiment, the gate oxide region has a common potential between thegate and the underlying body, which may eliminate gate oxide defectinduced leakage.

Generally, the boundary transistors connected as shown in FIG. 6 mayserve to electrically isolate the gate array cell 600 from adjacentcells. For example, coupling the gates of the boundary P-fets 615 and660 to power supply V_(DD) 610 switches both P-fet 615 and P-fet 660 offand electrically isolates the remaining P-fets in the collection ofP-fets 640. Similarly, coupling the gates of the boundary N-fets 620 and670 to power supply ground 630 switches N-fet 620 and N-fet 670 off,isolating the remaining N-fets in the collection of N-fets 650.Connecting the boundary transistors in this manner may allow the othertransistors in the gate array cell to be biased in the manner describedabove to help minimize or reduce yield losses resulting from gate oxidedefects. To minimize or attenuate drain-to-source leakage associatedwith connected boundary devices, alternative embodiments may beemployed.

FIG. 7 may depict a physical layout and arrangement, including metallayer interconnects, of the elements of gate array 600 shown in theschematic of FIG. 6. Similar to the components and arrangements shownfor gate array 500 in FIG. 5, gate array 700 has a collection ofpolysilicon gate structures 705 above an N-well 703, wherein the N-well703 surrounds the region around and below a P-diffusion area 710. Gatearray 700 also has a collection of polysilicon gate structures 750 abovea P-well, or substrate, and an N-diffusion region 745.

Gate array 700 may have a power supply rail voltage V_(DD) 715 coupledwith a power supply rail 720. Power supply rail 720 may traverse boththe collection of polysilicon gate structures 705 and the underlyingP-diffusion area 710. Additionally, power supply rail 720 carries thepower supply rail voltage V_(DD) 715 to the P-diffusion area 710 by wayof contacts 725 and 760.

Gate array 700 may have a power supply ground rail 740 passing over thecollection of polysilicon gate structures 750 and N-diffusion region745, coupling power supply ground potential 735 to N-diffusion region745 using contacts 730 and 786. Gate array 700 may also have the fourcontacts, namely contacts 725, 760, 730, and 786, located in closeproximity to the boundary of gate array cell 747.

Unlike gate array 500, gate array 700 has been configured and fittedwith numerous metal structures connecting various circuit elementswithin gate array 700, as well as routing the power supply rail voltageV_(DD) 715 and power supply ground potential 735 to circuit elements. Inparticular, metal structures 727 and 762 may be connected with powersupply rail 720 in order to couple the power supply rail voltage V_(DD)715 to the collection of polysilicon gate structures 705 by way ofassociated contacts. The collection of polysilicon gate structures 705may be the gates for the collection of P-fets shown in FIG. 6.

Metal structures 729 and 782 may be connected to power supply groundrail 740, coupling power supply ground potential 735 to the collectionof polysilicon gate structures 750. By way of vertical metal structure780, metal structure 761 may be coupled to power supply ground potential735 with portions of the P-diffusion region 710. In like fashion, metalstructure 785 may be indirectly connected to power supply rail 720 byway of vertical metal structure 727 and vertical metal structure 742 inorder to couple the power supply rail voltage V_(DD) 715 to portions ofthe N-diffusion region 745. Stated in simple terms, additional metalstructures or interconnects such as 727, 728, 729, 742, 761, 762, 765,768, 780, 781, 782, and 785, may be added to an uncommitted gate array,similar to unconfigured gate array 500 depicted in FIG. 5, in order tocreate the capacitive devices shown in FIG. 2B and FIG. 3B. Theseadditional metal layer interconnects may also be used to isolate thenewly created capacitive devices similar to the isolation methodsdiscussed for FIG. 6. Worth emphasizing, one may define select sectionsof metallic interconnect as having relatively narrow dimensions to actas fusible links should a short occur within a gate array decouplingcapacitor personalization region, such as a short from V_(DD) to ground.The embodiment of FIG. 7 contains vertical metal structures defined tohave relatively narrow widths, namely metal structures 727, 729, 742,762, 780, and 782. Other embodiments may not have such fusible links,with the vertical metal structures defined with different dimensions.

As illustrated in FIG. 7, one may increase the device capacitance byarranging power rails and/or metal structures near the device. Forexample, one may increase capacitance of the device by interleavingfirst metal layer structures coupled to power supply V_(DD) and ground.In FIG. 7, power supply rail 720 couples the power supply rail voltageV_(DD) 715 with metal structures 765, 768 and 785 by way of metalstructures 727, 762 and 742. Similarly, power supply ground rail 740couples power supply ground potential 735 to metal structures 728, 781,and 761 by way of metal structures 729, 782, and 780. The horizontalmetal structures coupled to power supply rail voltage V_(DD) 715 areeach separated by horizontal metal structures coupled to power supplyground potential 735. For example, metal structure 761, coupled to powersupply ground rail 740, separates power supply rail 720 and metalstructures 765 and 768, all three coupled to power supply rail voltageV_(DD) 715. In like fashion, metal structure 765, coupled to powersupply rail voltage V_(DD) 715, separates metal structures 761 and 781,both coupled to power supply ground potential 735. Interleaving, oralternating, metal structures in this fashion may increase capacitanceof the device. Additionally, reducing the space between these horizontalmetal structures and increasing the sidewall or lateral area may bolsterthe overall capacitance of the device.

Turning now to the next figure, FIG. 8 illustrates an alternativeembodiment wherein a gate array 800 may be configured to providecapacitance while minimizing the effects of gate oxide defects anddrain-to-source leakage. Stated generally, P-fet and N-fet structuresnear the gate array 800 boundaries may be connected in series and biasedto an off state, which may significantly reduce drain-to-source leakage.

Stated more specifically, boundary P-fet 810 and adjacent P-fet 825 maybe connected in series, with the source of boundary P-fet 810 coupled tothe drain of P-fet 825 and the source of adjacent P-fet 825 coupled to apower supply ground connection 835. As stated before, the drainconnection of boundary P-fet 810 may be connected to a power supplyvoltage V_(DD) 805 due to the gate array 800 cell definition. To isolatethis power supply potential from the rest of the gate array 800collection of P-fets 845, the gate of boundary P-fet 810 and the gate ofadjacent P-fet 825 may both be coupled to power supply voltage V_(DD)805. Coupling power supply voltage V_(DD) 805 to the gates of boundaryP-fet 810 and adjacent P-fet 825, which are connected in series, mayturn off boundary P-fet 810 and adjacent P-fet 825 and significantlyreduce the magnitude of drain-to-source leakage current flowing betweenthe source of adjacent P-fet 825 and the drain of boundary P-fet 810.

In the same manner, opposite boundary P-fet 865 and opposite adjacentP-fet 860 may be connected in series and turned off by connecting thegates of both transistors to power supply voltage V_(DD) 805. Connectingthe transistors in this fashion may also help reduce the magnitude ofdrain to source leakage current flowing between the source of oppositeadjacent P-fet 860 and the drain of opposite boundary P-fet 805.

Just as the magnitude of the drain to source leakage may be reduced forthe collection of P-fets 845, the same may be accomplished for the gatearray 800 collection of N-fets 850. The outer boundary N-fets, namelyN-fet 815 and N-fet 875, may be connected in series with adjacent N-fets830 and 870, respectively, and have all their gates connected to powersupply ground connection 835. Connecting the N-fets in this manner willreduce the magnitude of the drain to source leakage current flowingbetween the source of N-fet 815 and the drain of N-fet 830, as well asthe current flowing between the source of N-fet 875 and the drain ofN-fet 870. In alternative embodiments, one may connect more than twoP-fets and N-fets in series in an effort to reduce leakage current evenfurther.

Similar to other embodiments, the collection of P-fets 845 and thecollection of N-fets 850 may be configured as capacitive structures. Inthe case of the collection of P-fets 845, all of the P-fets notconfigured for boundary isolation may be have their sources and drainscoupled to power supply ground connection 835 and their gates coupled topower supply voltage V_(DD) 805. The collection of P-fets 845 used tocreate capacitance are all connected in parallel. Likewise, thecollection of N-fets 850 used to create capacitance are also allconnected in parallel. Capacitance is formed by the diffusion-to-bodyjunction capacitances and the gate-to-source and gate-to-draincapacitances.

A physical implementation of the gate array 800 on a semiconductorsubstrate may be configured similar to the array shown in FIG. 9. FIG. 9illustrates one physical arrangement, or configuration, of metal layersand contacts that may be used to connect the various components to forma capacitive apparatus like the gate array 800 shown in FIG. 8. FIG. 9depicts a gate array cell 950 comprising a collection of polysilicongate structures 955 positioned over an N-diffusion region 945 and acollection of polysilicon gate structures 905 positioned over aP-diffusion region 910 and N-well 903. A power supply rail 920 may crossthe collection of polysilicon gate structures 905 and run parallel withthe P-diffusion region 910, supplying power supply voltage V_(DD) 915 tothe P-diffusion region 910 boundaries, through boundary contact 925 andboundary contact 960, and other areas where power supply voltage V_(DD)915 may be needed.

Conversely, a ground supply rail 940 may cross the collection ofpolysilicon gate structures 955 and run parallel with the N-diffusionregion 945, supplying power supply ground 935 to the N-diffusion region945, by boundary contact 932 and boundary contact 986, as well as otherareas requiring power supply ground 935. As depicted in FIG. 9,additional metal interconnects may be added to transform an uncommittedgate array into a capacitive apparatus, like the embodiment shown inFIG. 8.

By examining FIG. 9 and referring back to FIG. 8, one may see that afirst metal interconnect 926 and a second metal interconnect 962 may beadded to couple power supply voltage V_(DD) 915, through a number ofcontacts, to the collection of polysilicon gate structures 905, whichare the P-fet gates of gate array cell 950. Similarly, a third metalinterconnect 929 and a fourth metal interconnect 985 may be added tocouple power supply ground 935, through a number of contacts, to thecollection of polysilicon gate structures 955, which are the N-fet gatesof gate array cell 950. Referring back to FIG. 8, these connections mayprovide the connections for power supply voltage V_(DD) 805 to the gatesof the collection of P-fets 845. Likewise, these connections may createthe numerous power supply ground connections 835 coupled to the gates ofthe collection of N-fets 850.

Vertical metal interconnect 970 may be added to couple power supplyground 935 to the P-fet sources and drains in the P-diffusion region910. Likewise, vertical metal interconnect 927 may be added to couplepower supply voltage V_(DD) 915 to metal structure 980 and theassociated N-fet sources and drains in the N-diffusion region 945.Referring back to FIG. 8, these ground and source connections maycorrespond to power supply ground connections 835 for the collection ofP-fets 845 and power supply voltage V_(DD) 805 connections for thecollection of N-fets 850, respectively.

As noted for the metallurgical connections in FIG. 7, one should alsonote that select sections of the metallic interconnect may be defined tohave minimum wire widths which may act as fusible links should a V_(DD)to ground short should occur within the gate array decoupling capacitorpersonalization region. In the embodiment of FIG. 9, vertical sectionsof metal 926, 927, 929, 962, 970, and 985 have been defined with minimumwire widths.

As was noted for the metallurgical arrangement of FIG. 7, one mayincrease overall device capacitance by interleaving metal structurescarrying different voltage potentials. In FIG. 9, metal structurescoupled to power supply voltage V_(DD) 915, namely metal structures 920,964, 968, and 980, are separated by interleaving metal structures 963,928, 975, and 940, which are coupled to power supply ground 935. Thespacing between these metal structures may generally be decreased inorder to increase the sidewall or lateral capacitance and bolster theoverall decoupling of this gate array capacitive apparatus.

FIG. 10 depicts a flowchart 1000 of an embodiment to form a capacitiveapparatus in a semiconductor substrate for an integrated circuit byarranging elements of a gate array. Flowchart 1000 begins with creatinga gate array containing a plurality of P-fets and a plurality of N-fetsin a semiconductor design (element 1010). In different embodiments, thegate array may contain a variety of different circuit elements andcombinations of those elements. For example, the gate array may onlycontain a number of N-fets, without any P-fets. Conversely, in otherembodiments, the gate array may only contain a number of P-fets, withoutany N-fets. Additionally, the type of P-fets and N-fets may vary indifferent embodiments. For example, the gate array in some embodimentsmay comprise N-fets in one or more P-wells. The gate array may alsocomprise elements other than simple N-fets and P-fets. For example, thestructure may contain more complex elements, such as NAND gates.However, for the sake of clarity and understanding in this description,one may assume that the gate array selected for illustrating FIG. 10 hasa collection of P-fets located in an N-well and a collection of N-fetslocated in a P-well.

After creating the gate array containing the plurality of P-fets and theplurality of N-fets (element 1010), the sources and the drains for twoor more P-fets in the gate array may be coupled to a ground potential ofa power supply (element 1020) and the gates of the P-fets may be coupledto a positive voltage of the power supply (element 1030), to create acapacitive apparatus. Since these two or more P-fets may be used tostore electrical energy, they may be referred to as capacitance P-fets.In alternative embodiments, capacitance may be created from differentP-fet configurations. For example, instead of coupling the sources andthe drains of the P-fets to ground, they may instead be coupled to thepositive power supply voltage while the P-fet gates are coupled toground. The method of arranging and connecting the structure elementsmay depend on a variety of factors, such as the type of gate arraystructures available or the magnitude of capacitance needed.

Analogous to connecting P-fet elements for capacitance, N-fets may beconnected for capacitance. The sources and the drains for two or moreN-fets in the gate array may be coupled to the positive voltage of thepower supply (element 1040), while the gates for the two or more N-fetsmay be coupled to the ground potential of the power supply (element1050) to create a capacitive apparatus. Similar to the different waysP-fets may be configured for capacitance, in alternative embodiments theN-fets may also be configured differently. For example, one may couplethe N-fet gates, sources, and drains with the positive voltage of thepower supply while grounding the associated P-well, to createcapacitance.

Depending on the gate array configuration used, a circuit designer mayisolate the P-fet and N-fet capacitive apparatuses from the rest of thecircuit. Such a measure may be necessary or desirable to reduce theeffects of leakage currents when, for example, the gate array cell hasfunctionally committed circuit elements that facilitate rapid circuitdesign, such as predefined routing of power supply voltage and groundrails that may be preconfigured to connect with diffusion materials. Thecapacitance P-fets in the gate array may be isolated from the rest ofthe circuit by coupling two boundary P-fets in series (element 1060) andreverse biasing the P-fets by coupling the gates of the two boundaryP-fets to VDD (element 1070). These P-fets used for isolation may bereferred to as boundary P-fets, because the boundary P-fets may belocated separately from the capacitance P-fets, yet coupled to thecapacitance P-fets via a common source connection.

In an alternative embodiment, a single reverse-biased boundary P-fet maybe used to isolate the capacitance P-fets. In other embodiments, threeor more series-connected boundary P-fets may be reverse-biased toisolate the capacitance P-fets. In further embodiments, variouscombinations of single and multiple series connected boundary P-fets maybe coupled on each boundary of the capacitance P-fets to isolate thecapacitance structure.

The capacitance N-fets in the gate array may be isolated from the restof the circuit by coupling two boundary N-fets in series (element 1080)and reverse biasing the N-fets (element 1090) by coupling the gates ofthe boundary N-fets to ground. Similar to the boundary P-fets, theseboundary N-fets may be located separately from the capacitance N-fetsand still be coupled to the capacitance N-fets via a common drainconnection. In different embodiments, various combinations of single andmultiple series connected boundary N-fets may be coupled to thecapacitance N-fets to isolate the capacitance structure.

FIG. 11 depicts a flowchart 1100 illustrating a process of couplingfield effect transistors in such a manner to store and dischargeelectrical energy in an integrated circuit. The process begins byapplying power to an integrated circuit from a power supply (element1110). After applying power to the IC, energy may be transferred fromthe power supply to gate array FETs configured as a capacitive apparatus(element 1120). The number of FET structures used to create capacitancefor the circuit may vary from one FET structure to numerous structures.Additionally, the FET structures may be N-fets, P-fets, or combinationsof both N-fets and P-fets. In some embodiments the FETs may be N-fets,with the sources and the drains being coupled to a positive potential ofthe integrated circuit power supply and the gates being grounded. Insome embodiments, the FETS may be P-fets with the sources and drainscoupled to ground, while the gates of the P-fets are coupled to thepositive potential.

The process of FIG. 11 continues by storing energy in the diffusionmaterial of the gate array FETs (element 1130). In various embodiments,this storing of energy in the diffusion material of the FET structuremay occur during a period the integrated circuit is temporarily insteady state, or when the integrated circuit is operating at a reducedfrequency. The FET capacitance structure may discharge or relinquishenergy stored in the structure (element 1140) and transfer thedischarged energy to a different integrated circuit component or device(element 1150). In various embodiments, such discharge of stored energymay occur during transitional periods or periods when the integratedcircuit operates at increased rates, which consume more energy. Forexample, the FET capacitance structure may discharge energy to anadjacent memory module when the integrated circuit temporarily operatesat an increased frequency. This process of using the gate array FETcapacitance structure to store and discharge energy may continue as longas the IC continues to operate (element 1160).

It will be apparent to those skilled in the art having the benefit ofthis disclosure that the present invention contemplates methods andapparatuses for creating capacitance structures in a semiconductorsubstrate of an integrated circuit, with reduced issues of gate oxidedefects and leakage currents. It is understood that the form of theinvention shown and described in the detailed description and thedrawings are to be taken merely as examples. It is intended that thefollowing claims be interpreted broadly to embrace all the variations ofthe example embodiments disclosed.

Although the present invention and some of its advantages have beendescribed in detail for some embodiments, it should be understood thatvarious changes, substitutions and alterations can be made hereinwithout departing from the spirit and scope of the invention as definedby the appended claims. Further, embodiments may achieve multipleobjectives but not every embodiment falling within the scope of theattached claims will achieve every objective. Moreover, the scope of thepresent application is not intended to be limited to the particularembodiments of the process, machine, manufacture, composition of matter,means, methods and steps described in the specification. As one ofordinary skill in the art will readily appreciate from the disclosure ofthe present invention, processes, machines, manufacture, compositions ofmatter, means, methods, or steps, presently existing or later to bedeveloped that perform substantially the same function or achievesubstantially the same result as the corresponding embodiments describedherein may be utilized according to the present invention. Accordingly,the appended claims are intended to include within their scope suchprocesses, machines, manufacture, compositions of matter, means,methods, or steps.

1. An apparatus, comprising: a plurality of field effect transistors ina semiconductor substrate, wherein the plurality of field effecttransistors comprise a gate array of an integrated circuit; a source anda drain coupled to a metal trace for a first voltage, wherein the sourceand the drain are of a first transistor in the plurality of field effecttransistors; a gate of the first transistor coupled to a metal trace fora second voltage; and a channel region of the first transistor coupledto the metal trace for the second voltage, wherein the coupling of thesource and the drain to the metal trace for the first voltage and thecoupling of the gate and the channel region to the metal trace for thesecond voltage enable the first transistor to function as a capacitorfor the integrated circuit.
 2. The apparatus of claim 1, wherein theplurality of field effect transistors are P-type field effecttransistors, the first voltage being negative relative to the secondvoltage.
 3. The apparatus of claim 2, further comprising a second drainand a second gate of a boundary transistor coupled to the metal tracefor the second voltage, and a second source of the boundary transistorcoupled to the metal trace for the first voltage, to electricallyisolate the capacitive device from an external integrated circuit. 4.The apparatus of claim 2, further comprising a second drain and a secondgate of a first boundary transistor coupled to the metal trace for thesecond voltage, a second source of the first boundary transistor coupledto a third drain of a second boundary transistor, a third gate of thesecond boundary transistor coupled to the metal trace for the secondvoltage, and a third source of the second boundary transistor coupled tothe metal trace for the first voltage.
 5. The apparatus of claim 1,wherein the plurality of field effect transistors are N-type fieldeffect transistors, the first voltage being positive relative to thesecond voltage.
 6. The apparatus of claim 5, further comprising a secondsource and a second gate of a boundary transistor coupled to the metaltrace for the second voltage, and a second drain of the boundarytransistor coupled to the metal trace for the first voltage, toelectrically isolate the capacitive device from an external integratedcircuit.
 7. The apparatus of claim 5, further comprising a second sourceand a second gate of a first boundary transistor coupled to the metaltrace for the second voltage, a second drain of the first boundarytransistor coupled to a third source of a second boundary transistor, athird gate of the second boundary transistor coupled to the metal tracefor the second voltage, and a third drain of the second boundarytransistor coupled to the metal trace for the first voltage.
 8. Theapparatus of claim 1, wherein the capacitor for the integrated circuitcomprises a decoupling capacitor.
 9. A system, comprising: a processorformed in a semiconductor substrate, wherein the processor is coupledwith random access memory; a plurality of field effect transistors inthe semiconductor substrate, wherein the plurality of field effecttransistors comprise a gate array of an integrated circuit; a source anda drain coupled to a metal trace for a first voltage, wherein the sourceand the drain are of a first transistor in the plurality of field effecttransistors; a gate of the first transistor coupled to a metal trace fora second voltage; and a channel region of the first transistor coupledto the metal trace for the second voltage, wherein the coupling of thesource and the drain to the metal trace for the first voltage and thecoupling of the gate and the channel region to the metal trace for thesecond voltage enable the first transistor to function as a capacitorfor the integrated circuit.
 10. The system of claim 9, wherein theplurality of field effect transistors are P-type field effecttransistors, the first voltage being negative relative to the secondvoltage.
 11. The system of claim 10, further comprising a second drainand a second gate of a boundary transistor coupled to the metal tracefor the second voltage, and a second source of the boundary transistorcoupled to the metal trace for the first voltage, to electricallyisolate the capacitive device from an external integrated circuit. 12.The system of claim 10, further comprising a second drain and a secondgate of a first boundary transistor coupled to the metal trace for thesecond voltage, a second source of the first boundary transistor coupledto a third drain of a second boundary transistor, a third gate of thesecond boundary transistor coupled to the metal trace for the secondvoltage, and a third source of the second boundary transistor coupled tothe metal trace for the first voltage.
 13. The system of claim 9,wherein the plurality of field effect transistors are N-type fieldeffect transistors, the first voltage being positive relative to thesecond voltage.
 14. The system of claim 13, further comprising a secondsource and a second gate of a boundary transistor coupled to the metaltrace for the second voltage, and a second drain of the boundarytransistor coupled to the metal trace for the first voltage, toelectrically isolate the capacitive device from an external integratedcircuit.
 15. The system of claim 13, further comprising a second sourceand a second gate of a first boundary transistor coupled to the metaltrace for the second voltage, a second drain of the first boundarytransistor coupled to a third source of a second boundary transistor, athird gate of the second boundary transistor coupled to the metal tracefor the second voltage, and a third drain of the second boundarytransistor coupled to the metal trace for the first voltage.